1. Field of the Invention
This invention relates to a new catalyst composition useful for demetalation and desulfurization of oil stock, e.g. residua. More particularly, it relates to a specific catalyst composition of cobalt-molybdenum deposited magnesium aluminate spinel useful in a process for residua demetalation and desulfurization, said process comprising contacting said residua with said catalyst of cobalt-molybdenum deposited magnesium aluminate spinel in the presence of hydrogen.
2. DESCRIPTION OF THE PRIOR ART
Residual petroleum oil fractions containing relatively high proportions of metals, such as those heavy fractions produced by atmospheric and vacuum crude distillation columns, would represent excellent charge stocks for a cracking process were it not for their high metals content. Principal metal contaminants are nickel and vanadium, with iron and copper also sometimes present. Additionally, trace amounts of zinc and sodium may be present. Since these metals, when present in crude oil, are associated with very large hydrocarbon molecules, the heavier fractions produced by crude distillation contain substantially all the metals present in the crude, such metals being particularly concentrated in the asphaltene residual fraction. The metal contaminants are typically large organometallic complexes such as metal porphyrins.
At present, cracking operations are performed on petroleum fractions lighter than residua fractions. Typical cracking charge stocks are coker and/or crude unit gas oil, vacuum tower overhead, etc., the feedstock having an API gravity range of between about 15 and about 45. Since these charge stocks are lighter than residual hydrocarbon fractions, such residual fractions being characterized as having an API gravity of less than about 25, they do not contain significant proportions of the heavy and large molecules in which the metals are concentrated.
When metals are present in a cracking unit charge stock, such metals are deposited on the cracking catalyst. The metals act as a catalyst poison and greatly decrease the efficiency of the cracking process by altering the catalyst so that it promotes increased hydrogen production.
Sulfur is also undesirable in a cracking unit charge stock. The sulfur contributes to corrosion of the unit's mechanical equipment and creates difficulties in treating products and flue gases. At typical cracking conversion rates, about one-half of the sulfur charge to such a unit is converted to H.sub.2 S gas which must be removed from the gasoline product, usually by scrubbing with an amine stream. A large portion of the remaining sulfur is deposited on the cracking catalyst itself. When the catalyst is regenerated, at least a portion of this sulfur is oxidized to form SO.sub.2 or SO.sub.3 gas which must be removed from the flue gas which is normally discharged into the atmosphere.
In the past, high molecular weight, e.g. residual, stocks containing sulfur and metals have often been processed in a coker to effectively remove metals and also some of the sulfur. However, there are limits to the amount of metals and sulfur which can be tolerated in the product coke if it is to be marketable. Hence, there is a considerable need to develop economically practicable means for effecting the removal and recovery of metallic and sulfur contaminants from high boiling fractions of petroleum oils so that conversion of such contaminated oils to more desirable products may be effectively accomplished. The present application is particularly concerned with providing a catalyst for the removal of metal and sulfur contaminants from residual oil.
It has been proposed to improve the salability of high sulfur and metal content residual-containing petroleum oils by a variety of hydroprocessing methods, e.g. hydrodesulfurization and hydrodemetalation. However, difficulty has been experienced in achieving a commercially feasible catalytic hydroprocessing process. Short catalyst life in such processes is manifested by inability of a catalyst to maintain a relatively high capability for desulfurizing charge stock with increasing quantities of coke and/or metallic contaminants deposited thereon which act as catalyst poisons. Satisfactory catalyst life can be obtained relatively easily with distillate oils, but is especially difficult to obtain in desulfurizing residual oils, since the asphaltenic or porphyrinic components of an oil, which tend to form disproportionate amounts of coke, are concentrated in the residual fractions of a petroleum oil, and since a relatively high proportion of the metallic contaminants that normally tend to poison catalysts are commonly found in the asphaltene components of the oil. Further, on a commercial scale, these processes are rather costly due to high hydrogen consumption levels. It is, therefore, advantageous to provide a demetalation/desulfurization process catalyst such as the present invention which provides superior demetalation characteristics, good desulfurization benefits, low hydrogen consumption and satisfactory aging properties.
U.S. Pat. Nos. 3,716,479 and 3,772,185 propose demetalation of a hydrocarbon charge stock by contacting the charge stock with added hydrogen in the presence of a catalyst material derived from a manganese nodule.
U.S. Pat. No. 2,992,191 discloses broadly a calcined hydrodesulfurization catalyst consisting of a crystalline magnesium aluminate spinel which has associated therewith both cobalt and molybdenum. The critical parameters found by applicants to provide the improved catalyst material of the present invention are not taught or suggested by the patent specification.
British patent specification Nos. 1,318,941 and 1,318,942 disclose use of zinc, magnesium, beryllium or calcium aluminate spinels combined, after calcination, with a Group VIII metal, such as, for example, platinum, as a dehydrogenation catalyst.
Demetalation of hydrocarbon fractions is disclosed in U.S. Pat. No. 2,902,429 as contacting said fractions with a catalyst having a relatively small amount of a sulfur-resistant hydrogenation-dehydrogenation component disposed on a low surface area carrier, i.e. carrier with a surface area of not more than 15m.sup.2 /g, and preferably not more than about 3m.sup.2 /g. Examples of such low surface area carriers include diatomaceous earth, natural clays and Alundum.
There are numerous references in the art showing various metals combined with carriers such as alumina, silica, zirconia or titania as catalysts for use in demetalation and/or desulfurization processes. No references are known to the applicants which teach the present invention with its attendant benefits.